&ience & Global Security, 1994, Volume 4, pp.129-160Photocopying permit~ by license onlyReprints available directly from the publisher@ 1994 Gordon and Breach Science Publishers SAPrin~ in the Uni~ States of America

An Analysis of the NorthKorean Nodong Missile

David c. WrightO and Timur Kadyshevb

In this paper, we analyze the North KDrean missile program based on publicly avail-able information and a technical understanding of missile systems. In particular, wepresent models for the 1,000 kilometer-range Nodong missile and a 1,300 kilometer-range variant, both based on Scud technology. These models are single-stage missileswith four clustered Scud-engines and would have a circular error probable (CEP) oftwo to four kilometers or larger. We conclude that a 1,000 kilometer-range missile witha one tonne payload could be built using Scud technology. Moreover, it appears feasibleto extend the range to roughly 1,300 kilometers (with the same payload) if the missilebody can be constructed out of high-strength aluminum rather than steel, although itis unclear whether North Korea has such a capability. If both missiles are based onScud technology, their existence would not imply a breakthrough in North Korean mis-sile technology. These missiles would then represent essentially the longest-range mis-siles achievable without technically difficult steps such as multi-staging, suggestingthat future range increases may occur more slowly than past increases, depending onthe level of technical assistance North Korea can acquire. We look briefly at whatranges could be achieved using a two-stage model with Scud engines and the difficul-ties a missile such as the Nodong would present to endo-atmospheric missile defenses.

INTRODUCTION

Since the early 1990s there have been an increasing number of reports thatNorth Korea is developing a 1,000 to 1,300 kilometer-range missile, called theNodong in western press reports. North Korea's missile program has gener-ated international concern because of the country's proximity to South Koreaand Japan and its reported missile sales to Iran, Syria, and Libya. The signifi-cance of the Nodong missile program to several regions of the world becomes

a. Defense and Arms Control Studies Program, M.I. T., and the Union of ConcernedScientists, both of Cambridge, Massachusetts.

b. Senior Researcher at the Center for Program Studies of the Russian Academyof Sciences in Moscow, and affiliate of the Center for Arms Control, Energy andEnvironmental Studies, Moscow Institute of Physics and Technology.

,0 Wright and Kadyshev

clear if one considers the countries that become potential targets as the mis-sile range increases to 1,000 kilometers and beyond. In particular, a missilewith a range of 1,300 kilometers would give North Korea the capability toreach all of Japan, and Iran and Libya the capability to reach all of Israel.

At the same time, the extraordinarily closed nature of North Korean soci-ety has meant that little information is available about its missile program. Inparticular, there is little information to inform the public debate over thepotential threats and policy options.

In this paper, we present a technical analysis of the North Korean missileprogram and show that based on publicly available information and a techni-cal knowledge of missile systems it is possible to sketch a reasonably clear pic-ture of the program and to draw some general conclusions about possiblefuture developments. In particular, we present a model of the Nodong missileand consider the difficulties of producing the Nodong and longer-range mis-siles.

Our work is intended to answer the following questions. While NorthKorea has built missiles modelled on the 300 kilometer-range Soviet R-17/SS-lc "Scud-B" missile,. are reports of the Nodong credible, i.e., given what isknown about its missile program, is North Korea capable of building a 1,000kilometer-range missile? Is it possible to do so using Scud technology or wouldthe existence of such a missile indicate that North Korea has achieved thecapability to produce considerably more sophisticated missiles, either on itsown or with help from abroad? The former might indicate that the Nodong isapproaching the upper limit of what can be achieved without requiring thedevelopment of more sophisticated technology, such as multi-staging, and thatas a result future increases in missile range might occur relatively slowly. Onthe other hand, the latter might suggest that at least one such technologicalbarrier has been surmounted and future increases in range might occur rela-tively more rapidly. How credible are reports that North Korea has extendedor will soon extend the range of the Nodong to 1,300 kilometers? What mightbe the next steps in a program to develop missiles with even greater range?How effective might one expect tactical missile defenses to be against a missilesuch as the Nodong?

Our main conclusions follow largely from combining general physical prin-ciples with an understanding of Scud technology; while we have drawn onpublicly reported information about the North Korean missile program, ourconclusions are relatively insensitive to whether this information is correct in

.We identify Soviet and Chinese missiles by both their domestic and Western desig-nations. -

An Analysis of the North Korean Nodong Missile-

detail. The results draw heavily on calculations of the performance of missileshaving a range of characteristics and structuresl and information about thedevelopment of Soviet and Chinese missiles (especially the Soviet Scud-B).

Our goal is to provide a better understanding of the North Korean missileprogram and to present a set of plausible, self-consistent estimates ofparame-ters for the Nodong missile. Since the available information on the NorthKorean missile program is limited, the values of the parameters we presentare necessarily approximate, but are sufficient to answer some importantquestions about the missile.

THE HISTORY OF THE NORTH KOREAN MISSilE PROGRAM

Before analyzing the Nodong missile, we briefly describe the evolution of theNorth Korean missile program. This description will provide the basis forunderstanding the technology and knowledge base underlying the Nodongprogram. (Table 1 summarizes our estimates of the parameters of NorthKorea's missiles.)

DF-61 ProgramFrom 1976 to 1978 North Korea was involved in a Chinese program to designa missile having a range of 600 kilometers with a payload of one metric tonne(1,000 kilograms). This project was suspended after its main supporters wereousted from the Chinese government. The program built on the experienceChinese missile designers had acquired during the 1960s and 1970s whenthey designed missiles up through the DF-5/CSS-4. The proposed missile,called the DF-61, was intended to incorporate storable liquid fuel with highpressure turbo-pumps and inertial guidance.2

NKScud Mod-AFollowing the suspension of the DF-61 program, North Korea's next steptoward developing ballistic missiles was acquiring Soviet Scud-B missiles.3After reverse-engineering the Scud-B it reportedly began flight testing anindigenously produced version in 1984, which we refer to as the NKScud Mod-At,4 Like the Soviet Scud-B, the Mod-A is reported to have a range of 280 to

t We use "NKScud- to refer to North Korean missiles and distinguish them fromSoviet missiles. The terms "Mod-A, Mod-B, etc. will refer only to North Korean mis-siles.

32 Wright and Kadyshev~

Table 1: Approximate values of the parameters describing the missiles NorthKorea Is believed to have built or currently has under development. The entriesare estimates derived from publicly available data as described in the text.

Parameter Mod-A Mod-B Mod-C Nodong Extended-range

NodongFirst flight test 1984 1985 (?) 1990 1993 (?) -

Length(m) 11.25 11.25 12.55 15.5 15.5

Diameter (m) 0.88 0.88 0.88 1.3 1.3

Range (km)/ 300/ 340/ 500/ 1 ,CX:X:>/1 ,00) 1,300/1,00)payload (kg) 985 985 700

Dry booster mass (kg) 1.385 1,385 1,500 3,800.4,00) 2,800-3,CX:X:>

Propellant mass (kg) 4,CX:X:> 4,CX:X:> 5,CX:X:> 16,000 16,CX:X:>

Fuel fraction 0.74 0.74 0.77 0.80-0.81 0.84-0.85

Specific Impulse (5) 230 240 240 240 240

Burn time (s) 70 70 87.5 70 70

Thrust (N) 129,000 134,00) 134,000 540,CX:X:> 540,000

No. of engines 1 1 1 4 4

300 kilometers with a one tonne payload. This missile was probably used togain experience in producing missiles but was not deployed. The missile wasalmost certainly constructed using the same materials and design as theSoviet Scud-B. The lower curve in figure 1 shows the range versus payload fora missile with the characteristics of the Soviet Scud-B or the NKScud Mod-A(appendix A gives the details of the Scud-B model).

NKScud Mod-BBy 1985 North Korea reportedly began producing the Mod-B, an improvedversion of the Mod-A. Following its war with Iraq, Iran apparently agreed inthe mid-1980s to finance the development of this missile in return for NorthKorea supplying Iran with large numbers of the missile. Full-scale productionreportedly began in 1986 to 1987 with the first 100 missiles being transferred

~IIIII --.

An Analysis of the North Korean Nodong Missile

700

600

NKScud Mod-BIsp = 240 seconds

500

~ 400QI~ClIlJC EE ~

300 Soviet Scud BNKScud Mod-B

Isp = 230 seconds

200

100

00 200 400 600 800 1,000 1,200 1,400

payloadkilograms

Figure 1: Range-payload curves for a missile assuming a dry booster mass of 1,385 kilograms,propellant mass of 4,00) kilograms, burn time of 70 seconds, and a specific impulse (I~ of 230and 240 seconds. For a one tonne payload, these curves show ranges of roughly 300 and 340kilometers, respectively, for these two values of specific impulse. We identify the model with Isp= 230 seconds with the Soviet Scud-B and the NKScud Mod-A, and the model with Isp = 240seconds with the NKScud Mod-B.

to Iran in the fall of 1987.5The Mod-B is reported to have a range of 320 to 340 kilometers with a one

tonne payload-15 percent greater (about 40 kilometers) than the Mod-A-asa result of "minor modifications."S The most probable ways to achieve thisincrease would be to decrease the structural weight of the missile, to use amore energetic propellant, and/or to improve the engine to provide higher spe-cific impulse by, for example, increasing the temperature and pressure of thecombustion chamber. While it is possible that the North Koreans increasedthe size of the fuel tanks to carry more propellant, it seems more likely, giventhe modest range increase, that they would simply have used the same missilebody produced for the Mod-A.

Figure 1 shows that with no other changes in the missile, a 40 kilometerrange increase would require a decrease in payload of more than 150 kilo-grams. Alternatively, the payload could be maintained at one tonne if thestructural weight of the missile were decreased by this amount. Modernizingand miniaturizing the guidance system and using light-weight components inthe engine and fuel pump might lead to some weight savings, but probablyless than 150 kilograms. Another possibility, that they used high-strength alu-minum for the missile body instead of steel, is unlikely since it would probablyreduce the weight by 350 to 400 kilograms, which would lead to a considerablylonger range.6

The most likely explanation for the range increase of the Mod-B over theMod-A is improvements to the rocket engine that increase the specific impulse(possibly in addition to reductions in the mass of the missile). Figure 1 showsthat increasing the specific impulse from 230 to 240 seconds would giveroughly a 40 kilometer increase in range with no change in the missile mass.We will see that a value of 240 seconds appears to be consistent with the mod-els of the other North Korean missiles given below. Such a four percentincrease in specific impulse appears achievable, especially since North Koreais believed to have received assistance on engine design and production fromChina after Chinese missile engineers had considerable experience in produc-ing missiles.7 For example, North Korea could have upgraded the engines byusing high-pressure turbo pumps similar to those the Chinese had developedfor their DF-series missiles. Increasing the pressure and the temperature inthe combustion chamber by a small amount would lead to an increase in spe-cific impulse of the required magnitude.s

The circular error probable (CEP) of the Soviet Scud-B and NKScud Mod-B is reported to be 450 to 1,000 meters at a range of 300 kilometers.9

NKScud Mod-CAn obvious way to increase the range of the Mod-B is to lengthen the missileto carry more fuel, which is almost certainly what North Korea did to producea longer-range version, called the Mod-C. This method was used by Iraq toproduce the aI-Hussein missile that it used against Iran during the War of theCities and against Israel and Saudi Arabia during the Persian Gulf War.l0North Korean engineers were probably familiar with the details of this missilesince Iran is reported to have given North Korea access to aI-Hussein wreck-age from its war with Iraq.5 North Korean production of the Mod-C probablybegan during 1989 with the first known test launch in June 1990. North Koreareportedly began selling Mod-Cs to Iran and Syria in 1991.11

An Analysis of the North Korean Nodong Missile

800

700 , , , ,600 ' , , , , ,500 ' ,

....~ "..Ql QJ ..g' E 400 100 ..~- ..

~ ~~~

~

300 Isp = 230 sec.

200

100

00 200 400 600 800 1,000 1,200 1,400

payloadkilograms

Figure 2: Range-payload curves for a missile with a dry booster mass of 1.500 kilograms. pro-pellant mass of 5.00) kilograms. burn time of 87.5 seconds, and a specific impulse (Isp) of 230and 240 seconds. For lop = 240 seconds this model gives a range of 500 kilometers for a 700kilogram payload and we identify it with the NKScud Mod-C.

Figure 2 shows the range-payload curves for a model of the Mod-C missilethat carries 25 percent more fuel than the Mod-B. (This model assumes thesame technology as the model used fQr figure 1; details are given in appendixA.) The range of the Mod-C is given as 500 to 600 kilometers with a 600 to 700kilogram payload.12 The figure shows that this range-payload capability corre-sponds well to a model of the Mod-C that uses a specific impulse of 240seconds, which is the value also suggested by our model of the Mod-B.

The estimated accuracy of the Mod-C depends on what assumptions aremade about its reentry. If the missile remains intact during reentry its accu-racy will be considerably better than if the body separates from the warhead,as was seen to happen frequently with aI-Hussein missiles during the GulfWar. Assuming it has an adequate static margin, an intact missile will lead to-

36 Wright and Kadyshev

increased accuracy since it will help keep the warhead aligned with the air-flow rather than tumbling and will increase the weight and thus the speed ofthe warhead so that deflecting forces have less time to act. Assuming a ~EP of750 meters for the Mod-B we estimate a CEP of roughly 1,000 meters for anintact Mod-C and 1,300 to 2,600 meters for a separated warhead (see appendixB for details). The latter estimate assumes that the warhead is fairly stableduring reentry; if instead it tumbles or spirals significantly the CEP could beconsiderably larger. 13

Building a missile with range significantly greater than that of the Mod-Crequires substantial redesigning. Even increasing the amount of fuel to 50percent more than that of the Mod-B (while continuing to use a single Scudengine)14 would give a range of about 550 kilometers for a 700 kilogram pay-load-a range increase of only 10 percent over the Mod-C. Reducing the struc-tural weight of the missile by using aluminum rather than steel for the bodywould not produce a missile capable of flying 1,000 kilometers with a onetonne payload without a new, higher thrust engine.15

We argue below that if North Korea has built a substantially more capablemissile it is most likely based on Scud technology and materials but uses acluster of four Scud engines to increase the thrust. Such a configuration wouldbe relatively straightforward to achieve given North Korea's existing missileprogram, but it would represent essentially the upper limit of what could beachieved without developing a more powerful engine or new technical capabil-ities, such as multi-staging.

THE NODONG MISSILE

The Nodong missile:!: has been widely reported in the press as a new missilebeing developed by North Korea with an estimated range of at least 1,000 kilo-meters and capable of carrying chemical or nuclear warheads. It gained prom-inence in June 1993 after reports that North Korea had test flown the newmissile in late May.

Development of the Nodong is believed to have started in 1988 to 1989 andwas conducted in parallel with that of the Mod-C.16 A number of countrieshave apparently shown interest in the missile. There are reports that Libya is

* The Western designation of the missile is "Nodong" or "No Dong" after the name ofthe city where it was first observed. The transliteration of the North Korean name isnot unique and also appears as "Rodong," A number of sources refer to it as the"Nodong-l." Because of its characterization as a modified Scud, it is referred to as theMod-D in some sources.

11- .(r~~~

An Analysis of the North Korean Nodong Missile-

funding the development and will purchase the completed missiles and thatNorth Korea will help it set up a production facility.17 There are also reportsthat Iran is helping to fund the development of the missile18 and has negoti-ated to receive 150 Nodongs,19 possibly in return for oil shipments.20 Onereport states that Syria may be the first country to receive the Nodong once itis operational.21 Pakistani officials are also reported to have visited NorthKorea in 1992 to discuss the missile.22 As mentioned above, North Korea isreported to have sold Mod-C missiles to both Iran and Syria.

A defector from the North Korean military claims that in the late 1980s,North Korea began constructing four underground bases (two of which arecomplete) for launching long-range missiles against U.S. military bases inJapan and Guam.23

The May 1993 Flight TestThere are conflicting reports about attempted flight tests of the Nodong priorto the May 1993 test.24 In the May test, North Korea launched four missileson 29 and 30 May, of which only one or possibly two were Nodongs, with theremainder being Mod-Cs. All reports agree that the Nodongs flew only 500kilometers or less.25 The identification of some of the missiles as Nodongsrather than Mod-Cs is almost certainly based on the size of the missile derivedfrom monitoring by U.S. intelligence. North Korea has admitted that it testeda new missile but has not said whether the test was successful.26 The launchsite was given as Taepo-tong in Hwadae-kun, North Hamkyong Province, onthe east coast of North Korea,27 and the missile was reportedly fired from amobile launcher.28

The tests were monitored by the U.S. and Japanese military, and wereunusual for several reasons. First, the missiles were launched eastward acrossthe Sea of Japan toward Japan (see figure 3) unlike previous tests of the Mod-C, which were launched to the south.5 Second, the missiles did not send backtelemetry and North Korea did not announce, as is typically done, that it waslaunching a missile into air space and sea lanes used by commercial planesand ships. Some analysts have speculated that the tests were a demonstrationfor potential buyers rather than a serious technical evaluation.29

There are several indications that the 500 kilometer range of the flighttest was intentional and does not represent a failure of the missile. First, themissiles flew east toward Japan's Noto Peninsula. The Sea of Japan is only750 kilometers wide at that point and is therefore too narrow for a full-rangetest. Moreover, Japanese planes identified two North Korean ships moored fortwo days near the impact site of the tests, roughly 300 kilometers from the

~

An AnalysIs of the North Korean Nodong Missile

The Structure of the Nodong MissileWe present here the most likely structure for the Nodong missile. Our pro-posed structure results from combining technical information on the SovietScud-B, what little information has been reported about the Nodong missile,and what can be inferred from the history of the North Korean missile pro-gram and the histories of the Soviet and Chinese missile programs. Under-standing the Chinese missile program is especially useful since North Koreawas involved in the DF-61 missile development program with China in themid-1970s.

Reports of the Nodong give a range of 900 to 1,000 kilometers with a pay-load of 800 to 1,000 kilograms.3! Many reports also state that the range isexpected to be increased to 1,300 kilometers with a one tonne payload.32

The most probable configuration for the Nodong missile is that it uses acluster of four NKScud Mod-B engines to generate sufficient thrust. There area number of factors that favor this configuration over one using a single, morepowerful engine.33

First, clustering engines is a standard configuration-both the SovietUnion and China used clusters of four engines relatively early in their missileprograms. The first such missiles were the Soviet R-12/SS-4 (first tested in1955 and deployed in 1957) and the Chinese DF-3/CSS-2 (first successfullytested in 1966 and deployed in 1971).34 Since North Korea has a workingengine that it can produce and is well tested, it is likely to use a missile designbased on that engine, especially since setting up production of a new enginecould take considerable time and money.

Second, there are several technical reasons that favor clustering smallengines. For example, testing large engines is considerably more difficult andexpensive than testing smaller engines. In addition, experience shows thatclustering engines greatly reduces the mechanical vibrations from the enginessince the vibrations of the individual engines tend to destructively interferewith one another.35 Thus the stresses and the structural demands on thebooster are lower than if the missile used a single large engine.

Finally, building a single engine capable of the range-payload combinationof the Nodong would be a large extrapolation from their previous experience,requiring an engine with thrust four times that of the Scud engine.

Several sources have given rough dimensions of the N odong missile in therange of 15 to 16 meters in length and 1.2 to 1.3 meters in diameter.36 Thesefigures are almost certainly derived from remote imaging techniques and aretherefore approximate. However, if the assumption offour clustered engines iscorrect we would expect the fuel tanks to hold four times as much fuel as theScud-B, or roughly37 12.4 cubic meters. For a diameter of 1.3 meters, the

0 Wright and Kadyshev

QSoviet Scud-B NKScud Mod-C Nodong

NKScud Mod NB

Figure 4: Relative sizes of the Mod-B, Mod-C, and Nodong missiles. The lengths are 11.25meters, 12.55 meters, and roughly 15.5 meters, respectively. The diameters are 0.88 meters,0.88 meters, and roughly 1.3 meters, respectively.

length of the propellant tanks would be 9.3 meters (plus some extra spacebetween and at the ends of the tanks). Assuming the length of the section con-taining the engine below the propellant tanks is the same as in the Scud-B(two meters) and the section containing the guidance system and compressedair capsules between the warhead and the propellent tanks is increased from1.3 meters in the Scud-B to about two meters to accommodate the additionalair capsules required by the larger propellant tanks, one finds a booster lengthof roughly 13.5 meters (where we have included 0.2 meters between and at theends of the propellant tanks). Further assuming that the warhead section istwo meters long (see appendix C) gives a total length of 15.5 meters, whichagrees well with the reported length (see figure 4).

We estimate the shape and the ballistic coefficient of the warhead by con-

An AnalysIs of the North Korean Nodong Missile

sidering the heating of the warhead during reentry (see appendix C). Weassume that, as with the Scud-B, the warhead does not have an ablative coat-ing. A key assumption underlying our estimates of the heating and the accu-racy is that the body of the Nodong will separate from the warhead before thewarhead reaches deep into the atmosphere. Even if the warhead is notdesigned to separate, at 10 to 20 kilometers altitude the missile body wouldlikely disintegrate under the action of the atmospheric forces as almost alwaysoccurred with aI-Hussein missiles during the Iran-Iraq War and the 1991 GulfWar. Since the reentry speed of the full Nodong is much greater than that ofthe aI-Hussein, the peak atmospheric forces on the Nodong will be roughly 70percent larger, essentially assuring separation of the empty fuel tanks. As aresult, North Korea may have designed the warhead to separate from the bodybefore reentry since the process of breaking off the body can reduce the accu-racy of the missile. Separating the warhead does not require sophisticatedtechnology although if not done carefully it can cause the warhead to tumble,which will also degrade accuracy.

As discussed in appendix C, we estimate the ballistic coefficient of thewarhead to be roughly 36,000 to 48,000 N m-2 (750 to 1,000 Ib ft-2), sincelarger values would lead to very high heating of the warhead as a result of thehigh reentry speed of the Nodong.

Assuming the missile holds four times as much propellant as the Scud-Bgives a total propellant mass of 16 tonnes. We estimate a dry booster mass ofroughly 3,800 to 4,000 kilograms, which gives a fuel fraction of 0.80 to 0.81(see appendix D for details). Figure 5 shows range-payload curves for twomodels of the Nodong that use these masses, four clustered Scud engines, andthe same propellant and construction as assumed for the Mod-B and Mod-C(see appendix A). These models give ranges of 915 to 965 kilometers for a onetonne payload. Given the approximate nature of the information available,this is in excellent agreement with reported values and suggests that a similarmodel is the source of the intelligence estimates of the missile's range.

Extending the Nodong Range to 1,300 Kilometers'As noted above, a number of sources report that the range of the Nodong willbe increased to 1,300 kilometers with a one tonne payload. We discuss herehow such a range extension might be achieved. A Saudi Arabian newspaperhas reported that North Korea plans to test this missile in southeastern

, Some sources refer to this missile as the Nodong-2, although this name is usuallyreserved for a longer range missile that may be under development (see below). The1,300 kilometer-range missile would more appropriately be called the Nodong Mod-B.

~2 Wright and Kadyshev

1,800

, , , , ,1,600 , "" ',extended-range

, , Nodong, ,, ,, ,, ,, ,1,400 ' , ' ,, ,, ,, ,

',', fuel~ " ", fractionGI... ' ,CI ' ,c WE 1 200 ' , , '0.85IQ , ,~ a ,~ " 0.84

1,0000.810.80

800

00 200 400 600 800 1,000 1,200 1,400

payloadkilograms

Figure 5: Range-payload curves for models of the Nodong missile. The curves assume fourScud engines, a propellant mass of 16,(XX) kilograms, a burn time of 70 seconds, a specificimpulse of 240 seconds, and a fuel fraction (propellant mass divided by the full booster mass(including propellant» as given on the plot.

Iran.38 While North Korea denies such reports, the local geography restrictswhere North Korea could test such a missile (see figure 3) and may increaseits interest in using a foreign test site.

Figure 5 shows that part but not all of the 300 kilometer range increasecould result from reducing the payload below one tonne. However, we assumehere that in extending the range North Korea would attempt to keep the pay-load near one tonne to accommodate a potential future nuclear warhead.39

The main options for increasing the range are:

.building a new engine or modifying the existing engine to give greaterthrust,.

An AnalysIs of the North Korean Nodong Missile

.lengthening the missile to increase the amount of fuel, as was done to pro-duce the Mod-C, and/or

.reducing the structural weight of the missile.

We will argue that the third option is the most likely.Achieving a 300 kilometer increase in range would require increasing the

thrust of the Scud engine by 10 to 15 percent, which is probably too great anincrease to achieve by modifying the existing engine. On the other hand, ifNorth Korea decided to design and build a new engine, it seems unlikely itwould focus on such a moderate increase in performance. Moreover, a substan-tial redesign of the engine would likely require a substantial enough redesignof the missile that it would not be considered a modification of the Nodong.

Our calculations show that lengthening the missile and increasing theamount of propellant by 25 percent would only increase the range of the mis-sile by 100 to 150 kilometers, depending on the amount of structural weightadded to the booster. Even a 50 percent increase in propellant would onlyincrease the range by about 200 kilometers.

On the other hand, it appears to be possible to extend the range by 300kilometers while retaining a one tonne payload by reducing the structuralmass of the booster. We have assumed above that the Mod-B, Mod-C, andNodong missile bodies were made of steel. It is known, however, that alumi-num-magnesium alloy was used for the missile bodies of the Chinese DF-3/CSS-2 and DF-4/CSS-3 missiles, which were developed in the 1960s.40 IfNorth Korea had the capability to construct missiles out of a similar material,it is possible they could reduce the structural mass of the booster by roughlyone tonne (see appendix D). Range-payload curves calculated assuming fuelfractions of 0.84 and 0.85 show that this reduction would increase the rangeby roughly 300 kilometers while retaining a one tonne payload (see figure 5).

It is unclear whether there are indications that North Korea is actuallybuilding a 1,300 kilometer-range missile or whether it is a worst-case analysisconsidered by U.S. intelligence. Moreover, it not clear whether North Koreacould build such a missile, although the technical barriers do not seem partic-ularly large. While North Korea does not have a domestic airline industry thatwould give it experience in fabricating large bodies using aluminum alloys,the techniques are standard and well-known. It could presumably receivematerials and technical assistance from a number of countries, including Iran.

Estimates of Nodong AccuracyIt is likely that the Nodong's inertial guidance system uses gyros and body-

Wright and Kadyshev

mounted accelerometers. The control system almost certainly uses graphitevanes in the exhaust to divert the thrust, which is the method used in theScud-B as well as early Chinese missiles including the DF -3/CSS-2, which hadfour clustered engines.41

In estimating the accuracy of the Nodong we assume it uses Scud guid-ance technology and we scale up the errors of the Mod-B, as described inappendix B. As mentioned above, we assume that the warhead will separatefrom the missile body before it reaches low altitudes. We conservatively esti-mate that the CEP of the Nodong at 1,300 kilometers range will be 2,000 to4,000 meters (see appendix B). If the warhead tumbles or spirals significantlyduring reentry the CEP may be considerably larger. One source reports a fig-ure of 2,000 meters for the Nodong CEP;42 we believe this figure is probablytoo small. The total error for a missile has two main contributions: errors inplacing the missile on the proper ballistic trajectory at the end of boost phaseand errors from atmospheric buffeting during reentry. If, as is likely, the reen-try errors are the dominant contribution to the inaccuracy, then improving theguidance system of the missile would do little to improve the CEP since theguidance and control system only acts during the missile's boost phase andcan only reduce the first contribution above. Improving the accuracy wouldinstead require reducing the reentry errors, which is difficult.

Missile ReliabilityThe reliability of North Korea's missiles may be questionable since theexpense of conducting flight tests may limit its testing program and thus itsability to achieve confidence in the missile's performance. Moreover, some ana-lysts question the quality of workmanship on the missiles. Reports that NorthKorean missiles provided to Iran in the past were defective suggest that reli-ability has been a problem.43 On the other hand, some key components, suchas the engine, could be tested on the ground without requiring flight tests.

The reliability of the Nodong would be expected to be lower, perhaps sig-nificantly, than that of the Mod-B or Mod-C for a couple reasons. First, theNodong is larger and faster than previous missiles and is therefore subjectedto substantially higher stresses, which could lead to increased structural fail-ures. Second, the presence of four engines instead of one will multiply theprobability of a propulsion failure. For example, if individual Scud engines are95 percent reliable, a cluster of four would be only (0.95)4 = 81 percent reli-able; if instead the individual engines are 90 percent reliable, the reliability ofa cluster of four drops to 66 percent.

An Analysis of the North Korean Nodong Missile

THE NEXT STEP?

There are reports that North Korea is planning a Nodong-2 missile with a1,500 to 2,000 kilometer range.44 Such a range would require developingeither a considerably more powerful engine than the Scud-B engine, or multi-staging. Both methods have precedents. The Chinese DF -3/CSS-2 missile,which was first flown in late 1966, is a single-stage missile with four clusteredengines that produce enough thrust to achieve a reported range of 2,800 kilo-meters with a two tonne payload. The first Chinese two-stage missile, the DF-4/CSS-3 (first tested in 1970), used the DF-3 as the first stage and a single DF-3 engine to power the second stage.45 Both methods of extending the range aretechnically demanding, and the time required for North Korea to accomplishthem depends largely on what foreign assistance it is able to get.

To understand the extent to which multi-staging could increase the rangeof a missile that still used Scud engines, we consider a simple model of a two-stage missile that uses the Nodong as the first stage and a variant of the Mod-B as the second stage. Optimizing the size of the second stage for a Nodongfirst stage gives a second stage mass roughly equal to the mass of the Mod-B.Thus, we take the second stage to be essentially a Mod-B with the finsremoved. This configuration leads to a range of 1,750 to 1,800 kilometers for a1,000 kilogram payload.46 We emphasize that this estimate is crude andshould be considered only as suggestive.

CONCLUSIONS AND IMPLICATIONS

Several important conclusions follow from our analysis:First, it is possible to build a missile capable of flying 1,000 kilometers

with a one tonne warhead using Scud technology that North Korea is knownto produce. As a result, reports of the Nodong's development appear to be tech-nically credible. On the other hand, the current state of North Korean indus-trial capabilities has led to some skepticism that it could produce suchmissiles.47

Second, it appears feasible in principle to further increase the Nodongrange to around 1,300 kilometers with a one tonne warhead by constructingthe missile body from high-strength aluminum. We have no evidence, how-ever, to suggest whether this is actually being done or whether reports of sucha development represent a worst-case analysis by U.S. intelligence analysts.

If both the 1,000 and 1,300 kilometer-range missiles are based on Scudtechnology, their existence would not imply a breakthrough in North Korean

) Wright and Kadyshev

missile technology.Third, the Nodong missile is essentially the longest range missile that

North Korea could build with its existing level of missile technology. Furtherrange increases would require steps such as building a considerably morepowerful engine or using multiple stages-both of which are technically moredemanding than using existing components to build the Nodong. Thus, futurerange increases may occur relatively more slowly than in the past five years.However, the technical issues involved in both approaches are widely under-stood and the time required for North Korea to master them if it is interestedin doing so depends largely on what foreign assistance it can get.

Fourth, the accuracy of the Nodong is expected to be several kilometersand reducing this figure significantly would be very demanding. For example,if the errors are dominated by reentry errors rather than guidance and controlerrors, as appears likely, improving the guidance system of the missile wouldhave little effect on the accuracy. With such poor accuracy the N odong wouldnot be a militarily significant weapon if equipped with a conventional or chem-ical warhead but could be an effective terror weapon. Its ability to carry a onetonne payload could make it a more significant threat in the future if NorthKorea or Iran develop a deliverable nuclear weapon. However, even if armedwith a nuclear warhead its accuracy is so low that it still could not be usedagainst military point targets.

Finally, while some key missile components could be tested withoutrequiring flight tests, the overall reliability of the missiles may be low since itwould be expensive for North Korea to carry out an extensive flight testingprogram. An emerging nuclear state may be reluctant to trust such a missileto deliver one of its few nuclear weapons.

Implications for Tactical Missile DefenseThe possibility that a missile such as the Nodong could be operational in thenear future is already haVing important ramifications in number of countries,especially Japan and Israel. In particular, it has played a role in the debateover theater missile defense in both countries.48

The prospects for effective missile defense against such a missile usingendo-atmospheric missile defenses appear poor for several reasons, which wediscuss below. High altitude interceptors, such as THAAD, would face otherproblems, in particular decoys and debris clouds. THAAD is currently in earlystages of development; while "operational prototypes" may exist in the late1990s, current schedules do not call for a fully operational system until 2001.

The intrinsic inaccuracy of the Nodong makes it a perfect candidate for

An Analysis of the North Korean Nodong Missile

some of the countermeasures that worked successfully (albeit unintentionally)against the Patriot anti-missile system during the Gulf War.49 The Nodong'sinaccuracy results largely from unpredictable motions of its warhead causedby atmospheric forces during reentry. These motions place high demands onan interceptor since it must be able to generate extremely high lateral acceler-ations in order to change directions quickly enough to reorient toward the newposition of the warhead and intercept it. The same principle was used by theUnited States in developing its "evading MaRY," which was designed in the1970s to evade Soviet missile defenses.50 The low accuracy of the Nodong isevidence that it experiences strong atmospheric forces and severe buffeting.These forces could even be increased by intentionally adding a small asymme-try to the warhead (similar to the bent nose of the U.S. MaRY). The accuracyof the Nodong is low enough that the additional inaccuracy caused by such ameasure would be unimportant.

Moreover, increasing the speed of the incoming warhead would increaseits lateral accelerations and further increase the demands on a defensive sys-tem. Thus a second obvious countermeasure would be to increase the ballisticcoefficient of the warhead to increase its reentry speed.51 Such an increase canbe achieved simply by changing the shape of the warhead. The resultingincreased heating of the warhead would probably require an ablative heatshield to be added (see appendix B). Adding an ablative layer is not technicallydifficult if one is willing to accept some degradation in accuracy that wouldresult from asymmetric ablation. If the goal is to evade defenses, asymmetricablation may be a benefit since it would make the reentry forces even morevariable with time and thus make the warhead's path even less predictable.

Achieving even moderate intercept rates against such a missile would beextremely demanding and the costs of developing such a system must beweighed against the potential role they could play in reducing casualties anddamage. Missiles with conventional and even chemical warheads are unlikelyto cause high casualty rates against adequately prepared populations, unlessthey are used in very large numbers.52 As seen in the Gulf War, measures suchas adequate warning of attack and reinforced concrete construction can playamajor role in protecting the population. 53 On the other hand, any leakage rate

of the defensive system low enough to be acceptable against nuclear weaponsis probably unattainable. Even if the attacker had only a few nuclear war-heads it could launch a barrage of conventional and nuclear armed missiles tooverwhelm the defense.

Thus missile defenses do not offer a solution in the near term to the prob-lem of the development and potential use of missiles by North Korea or Iran.Other solutions should be explored and must be evaluated in terms of their

48

Wright and Kadyshev

feasibility, likely effectiveness, and cost. While diplomatic approaches toreducing these problems are extremely difficult and will likely have mixedsuccess, countries must be careful not to let a focus on missile defenses dis-tract them from addressing the underlying issues motivating these states.

APPENDIX A: TECHNICAL DETAILS OF THE NKSCUD MOD-A, B, AND C

MISSILES

The Soviet R-17/SS-lc "Scud-B" missile is 11.25 meters long, 0.88 meters in diameter,and has a launch weight of 6.37 tonnes when equipped with a 985 kilogram payload.54It is fueled by four tonnes of unsymmetrical dimethylhydrazine (UDMH) with inhib-ited red fuming nitric acid (lRFNA) as an oxidize~ and uses fuel pumps to move thefuel to the engines. The optimum mass ratio of oxidizer to fuel for this combination isroughly three and the volume ratio is 1.5;56 however, the actual ratios used by the mis-sile appear to be closer to 3.6 and 1.8.57 The latter values give a propellant volume of3.1 cubic meters.

Below the warhead section is a compartment containing the guidance system anda second compartment containing several canisters of compressed gas that is used todisplace the fuel and oxidizer from their tanks. The Scud is believed to use simple iner-tial guidance with three gyroscopes and body-mounted accelerometers, and has fourgraphite diverter vanes in the exhaust to control the flight path during boost phase.

With four tonnes of propellant the fuel fraction (the propellant mass divided by thetotal mass of the booster without the payload) is 0.74. Assuming a bum time of 70 sec-onds and a specific impulse of 230 seconds gives a thrust of 129,000 newtons.58 We cal-culate that this configuration (with a 985 kilogram payload) will have a maximumrange of 290 to 300 kilometers, which is roughly the quoted range of the Scud-B andthe NKScud Mod-A. Figure 1 shows the variation of range with payload for this missileassuming a specific impulse of 230 and 240 seconds.

We estimate the masses of various components of the missile as follows. Weassume that the walls are made of two to three millimeter thick steel, which has a den-sity of 7,800 kg m-3 and thus an area density of about 20 kg m-2. The outer surfacearea of the booster is roughly 25 m2, giving a mass of 500 kilograms. The ends of thefuel tanks and guidance compartment plus the fuel feed pipe would add roughly 125kilograms. The fins consist of two sheets of steel enclosing a lattice of supports plus thevanes for steering and the mechanism for controlling them. Each fin has a surface areaof about one square meter and we estimate their mass at 40 kilograms each. Weassume roughly 100 kilograms for the guidance system, 50 kilograms for the air canis-ters, and 200 kilograms for the engine, fuel pump, and related plumbing. Given thereported dry mass of the booster of 1,385 kilograms, this leaves roughly 250 kilogramsfor structural supports, etc.

We assume here that the NKScud Mod-C is essentially the same configuration asthe Iraqi aI-Hussein missile. The aI-Hussein is said to have been produced from theSoviet Scud-B by lengthening the fuel tanks by 1.3 meters59 to achieve a 25 percentincrease in fuel. Assuming, as above, that the missile body is made of two to three mil-limeters thick steel and that some supports would be added, this extension would

An Analysis of the North Korean Nodong Missile-

increase the booster mass by roughly 100 kilograms. Since the propellant mass wouldincrease to 5,000 kilograms, the booster mass (without payload) would be 6,500 kilo-grams, with a fuel fraction of 0.77. Range-payload curves for this model are shown infigure 2 assuming a specific impulse of 230 and 240 seconds.

APPENDIX B: ESTIMATING MISSILE ACCURACY

We estimate the accuracy of the Mod-C and Nodong missiles by scaling the errorsreported for the Mod-B to the longer ranges and higher reentry speeds. For this calcu-lation, we consider a Nodong with a 1,300 kilometer range.

The reported values for the circular error probable (CEP) of the Mod-B (and theSoviet Scud-B) ranges from 450 to 1,000 meter at 300 kilometer range. For our calcula-tion we assume a value of 750 meter, which is in the middle of this range. The CEP isrelated to the dispersion in impact location by:

CEP = 0.59 (DR + DXR) (B-1)

where DR and DXR are the average dispersions in the range and crossrange directions.Assuming DR = DXR = DB, we find DB = 640 m for the Mod-B.

The total dispersion of a missile results from two principle contributions: (1) guid-ance and control (G&C) errors, which lead to small errors in the missile velocity andorientation at booster burnout and (2) reentry errors. Since we do not know the relativesize of these two contributions for the Mod-B, we consider three cases: one in which thetwo contributions are equal, one in which dispersions resulting from reentry errorsdominate, and one in which dispersions resulting from G&C errors dominate. In thesecond two cases, we assume that the dominant error leads to dispersions three timesas large as the contribution arising from the other error. We also assume that reentryerrors and G&C errors are independent of one another so that the total dispersion isgiven by the square root of the sum of the squares of the component dispersions:

D = J (D(R» 2 + (D(GC» 2 (B-2)

where D<R) and D(GC) are the reentry and G&C dispersions, respectively.These assumptions allow us to calculate D(R) and D(GC) for the Mod-B for each of

the three cases. We then scale each of these dispersions separately to values appropri-ate to the Mod-C or Nodong and combine the resulting values to produce an estimate ofthe new total dispersion.

Guidance and Control DispersionsWe assume here that the guidance technologies used in the Mod-C and Nodong areessentially the same as in the Mod-B. To scale the guidance and control errors, we cal-culate the sensitivity of the missile's range to small changes in the vertical and hori-zontal components of the burnout velocity. These sensitivities, aRIiJV v and aRIiJV h, canbe calculated from the trajectory equations.GO We then use the values of these sensitiv-

50 Wright and Kadyshev-

ities appropriate to the Mod-B to determine the uncertainty in the components of theMod-B's burnout velocity, oVv and OVh, that leads to the assumed G&C dispersions for

the Mod-B:

D(GC) = oV ~dR~2-(dR~~)2+ (~ )2 (B-3)~ ldY:) + l~)

where we have assumed oV v = oV h S oV: Assuming that the guidance system will giveuncertainties in the burnout velocity of roughly this same size for the Mod-C andNodong, we can calculate the G&C dispersions for these missiles using equation (B-3)with values for the sensitivities appropriate to these missiles. Since the burnout speedof the Nodong is much greater than that of the Mod-B, oV for the Nodong may in real-ity be greater than that of the Mod-B. On the other hand, improvements in the guid-ance and control system will reduce oV: Nonetheless, the method outlined above willgive a rough estimate of the G&C errors for the Mod-C and Nodong.61 The values forthe three cases are given in table B-1.

Reentry DispersionsReentry dispersions result from aerodynamic forces acting on the warhead and have anumber of causes, including variations in atmospheric wind and density and a numberof "lift effects" that produce lateral forces on the reentering missile. For simple missilessuch as the Mod-B and Nodong the lift effects will dominate. The dispersions willresult from net lateral forces that are not averaged out during reentry.

The lift force experienced by a body travelling at speed V in an atmospheric den-sity p is given by:

1FLIFT = 2CLApy2 (B-4)

where CL is the lift coefficient, and A is the cross-sectional area of the warhead trans-verse to its direction of motion. Thus lift forces will be greatest for missiles travellingat high speeds, especially those with large values of ballistic coefficient (13) since theywill experience relatively low drag and will have large speeds low in the atmospherewhere p is large. Lift forces can have a number of causes, including asymmetries in thereentry vehicle and a misalignment between the direction of motion and the axis of thereentry vehicle (a non-zero angle of attack). In general, the lift force will fluctuate inmagnitude and direction during reentry; some of these fluctuations will tend to aver-age out, especially if the warhead is spinning, but the net, non-averaged lift forces willlead to unpredictable lateral motions of the warhead.62 We estimate the effect of thesenet forces in two ways, described below.

The first method we use to estimate reentry dispersions assumes that the magni-tude and the direction of the lift force are constant throughout reentry.63 To chose themagnitude OfCL' we calculate the trajectory of the Mod-B in the presence of such a liftforce and pick CL to give the v~ue D(R) assumed for the Mod-B. To estimate the lifteffects on a different missile, we then calculate the trajectory of that missile in thepresence of a net lift force using the same value of CLo

The second method assumes that the lift force varies randomly in magnitude and

An AnalysIs of the North Korean Nodong Missile

Table 8-1: Estimated CEPs for the Mod-C and Nodong missiles. 0

Missile D(GC) D(R) CEP

meters meters meters

Mod-B ~ 200 750

Mod-C (with body) 820 260 1,00)

Mod-C (warhead only) 820 720 1 ,300

Nodong (low 13) 1 A30 1210 2,200

Nodong (high 13) 1A30 1,00) 2,100

Mod-B 450 450 750

Mod-C (with body) 620 580 1 ,000

Mod-C (warhead only) 620 1,590 2,000

Nodong (low 13) 1,100 2,840 3,600

Nodong (high 13) 1,100 2,350 3,100

Mod-B 200 ~ 750

Mod-C (with body) 270 780 1,00)

Mod-C (warhead only) 270 2,180 2,~

Nodong (low 13) 480 3,900 4,~

Nodong (high 13) 480 3280 3,900

a. These tables HsI estimates for the guidance and control (G&C) ~ 000 reentry dispe($ions and the CEPs for theNKScud Mod-G and Nodong mlssaes, calculated as described n the text. 100 calculat1ons assume a CEP of 750 metersfor the NKScud Mod-B missile and the three cases correspond to different assumptions OOout how the totol dlspefsjonsof the Mod-B are divided between G&C and reentry dlspefsjons. (The Mod-B Is assumed to have II = 192.!XXJ N m-2

(4.!XXJ b ft-2». DIspefsjons are calcooted for four missile con~ratlons: (1) a Mod-G n ~h the missile remak"lS ntactfhrouglOUl reentry (II = 17B.!XXJ N m-2 (3,700 b ft-2». (2) a Mod-G n ~h the missile body Is assumed to separate andonly the warhead reenten <II = 55.!XXJ N m-2 (1,150 b ft-2», (3) a 1.300 kHometer-range Nodong missile with II = 36.!XXJ Nm-2 (750 b ft-2), and (4) a 1.300 kioo\eter-range Nodong missile with II = 48.!XXJ N m-2 (1.!XXJ b ft-2). VcMues of disper-Q)S are rounded to the nearest 10 meters and CEPs ae rounded to the neaest 100 meters. If the actua CEP of theMod-B Is larger or smaler than 750 meten. the estimates of the CEPs for the other missiles wiU Increase or decreaseaccordingly. These figures assume that the bo<ies remain roultllY aerodynamlcaty ~ during reentry; tumbling orpronounced spiraling of the warhead. or the presence of pieces of the missile body attached to the wahead, canlead to larger Kft forces and sI\7)iflcOnt1y larger dispersions.

>2 Wright and Kadyshev

direction throughout reentry 80 that the resulting dispersion is an average of this fluc-tuating force over the trajectory of the missile. Specifically, we calculated the trajectoryin the presence of a lift force acting in the plane of the trajectory, with the magnitude ofthe force multiplied by a number between -1 and 1 that changed randomly with eachstep of the numerical integration (every 0.1 seconds). An average dispersion was thencalculated by averaging over 2,000 runs. We calculated the trajectory of the Mod-B inthe presence of this fluctuating force and chose the magnitude ofCL to give the averagedispersion equal to the value of n(R) assumed for the Mod-B. This same value of CL,again weighted by a random number, was then used to calculate trajectories of theMod-C and Nodong, with an average dispersion calculated by averaging over 2,000such runs in each case.

These two methods give results that are within several percent of each other inmost cases, although in some cases they differ by 10 to 15 percent. The values for reen-try dispersions in table B-1 are the average of the results of the two methods.

Having calculated the reentry and G&C dispersions for the Mod-C and Nodong wecan calculate a total dispersion using equation (B-2), and a CEP using equation (B-1)(assuming that the crossrange dispersion will roughly equal the range dispersion wehave calculated). These CEPs are listed in table B-1.

For these calculations, we assume that the Mod-B has a 13 of 192,000 N m-2 (4,000lb ft-2).(which assumes a drag coefficient of 0.2). For the Mod-C, we calculate the CEPfor two cases: (1) the missile remains intact and the missile body does not separatefrom the warhead, which gives 13 = 178,000 N m-2 (3,700 lb ft-2), and (2) the body sepa-rates from the warhead, giving 13 = 55,000 N m-2 (1,150 lb ft-2) for the warhead (bothvalues assume a drag coefficient of 0.2). As explained in appendix C, we assume theNodong warhead will separate from the missile body and calculate the CEP for the twocases 13 = 36,000 N m-2 (750 lb ft-2) and 48,000 N m-2 (1,000 lb ft-2).

One way to reduce the reentry errors somewhat would be to increase the ballisticcoefficient of the warhead to increase its reentry speed.64 Significantly increasing thereentry speed, however, would greatly increase the atmospheric heating of the reentryvehicle and would almost certainly require adding an ablative coating to the reentryvehicle in order to allow it to withstand the greater heating. We assume that theNodong currently does not have such a coating. Adding a simple ablative coating maynot be technically demanding. However, such a coating must ablate smoothly and uni-formly or the ablation process will give rise to large lateral forces that will tend to off-set the gains in accuracy from the increased speed, and developing an advancedablative coating of this kind is very demanding.55 Thus even using such measureswould not decrease the CEP enough to make the Nodong a militarily significantweapon with a conventional warhead.

Finally, we note that the estimates of CEPs listed in table B-1 assume that theaxis of the warhead remains roughly aligned with its velocity during reentry. In thatsense these estimates are best-case estimates. If the warhead is not well aligned dur-ing reentry so that it undergoes pronounced spiraling, or tumbles, or if it experienceslarge lift forces from pieces of the missile body that remain attached after the bodybreaks off, the resulting dispersions can be much larger than those listed in the table.

An Analysis of the North Korean Nodong Missile",..,,~..~

APPENDIX C: ESTIMATING THE BALLISTIC COEFFICIENT AND LENGTH

OF THE NODONG WARHEAD

The heating of a reentering warhead increases with the ballistic coefficient and thereentry speed of the warhead:

QL ~ /3o.5~ QT ~ /3o.Sy2g48 (C-l)

where QL and QT are the total heat absorbed during reentry assuming the boundarylayer of air flowing past the warhead is laminar and turbulent, respectively; /3 is theballistic coefficient of the warhead; and VE is its reentry speed at high altitudes.66Be-low we use the expression for QT since the boundary layer is almost certainly turbulentin the regions of greatest heating (although for the values of interest both equationsgive similar results).

We use this equation to compare the heat absorbed by the Nodong to that absorbedby the aI-Hussein missile, under the assumption that the aI-Hussein was designed towithstand the reentry heating in the case in which the missile body remains attachedto the warhead during reentry. Using /3 = 178,000 N m-2 (3,700 Ib ft-2) and V E = 1,700m sec-l for the al-Hussein67 and /3 = 36,000 to 48,000 N m-2 (750 to 1,000 Ib ft-2) andVE = 3100 m sec-l for the Nodong, we calculate that the heating of the Nodong is 20 to50 percent greater than that of the al-Hussein.A value of/3 = 72,000 N m-2 (1,500 Ib ft-2) for the Nodong gives an absorbed heat of over twice that of the aI-Hussein. We there-fore take the ballistic coefficient of the Nodong to lie in the lower range of values.

As noted in the text, we assume that the Nodong warhead separates from the mis-sile body before it reaches low altitudes where most of the atmospheric drag and heat-ing occur, so that the mass of the reentering body is 1,000 kilograms. Values of /3 in therange 36,000 to 48,000 N m-2 (750 to 1,000 Ib ft-2) for a warhead with this mass and abase diameter of 1.3 meters correspond to values of the drag coefficient68 of 0.15 to 0.2.The drag coefficient can be related to the shape of the warhead using simple equa-tions;69 these relations give a warhead length of roughly two meters.

APPENDIX D: ESTIMATING THE BOOSTER MASS FOR THE NODONG

We use our estimates of the mass breakdown of the Mod-B in appendix A to estimatethe booster mass for the Nodong as follows. Since the weight of the propellant and theatmospheric forces on the booster will be greater than on the Mod-B, we assume thatthe body is made from a heavier gauge steel. We therefore assume three to four milli-meters of steel sheet metal with a mass per area of27 kg m-2. Using a booster length of13.5 meters and including the mass of the outside walls of the booster, the ends of thepropellant tanks and guidance compartment, and the fuel feed pipe, we calculate amass of roughly 1,800 kilograms. Since both the length and diameter of the missile arelarger than the Mod-B missile by about 50 percent, we assume the fins are also scaledup by this factor, which gives a mass of roughly 100 kilograms for each. We assume 800kilograms for the four engines plus associated pumps and plumbing, 100 kilograms forthe guidance package, and 200 kilograms for the air canisters. Scaling up the mass of

t Wright and Kadyshev

the structural supports, etc., from the Mod-B gives 500 to 700 kilograms, which leadsto a total dry booster mass of roughly 3,800 to 4,000 kilograms. This mass estimateleads to a fuel fraction of 0.80 to 0.81.

To estimate how much the missile's mass could be reduced by making the body outof aluminum, which has a density of 2,700 kg m-3, rather than steel, we assume theskin, fins, and some of the structural supports are made of aluminum. We assume thatthe skin is four to five millimeters thick aluminum sheet, having a density of roughly12 kg m-2, and that the number of structural supports may need to be increased. Usingthese assumptions the mass reduction might be 1,000 to 1,200 kilograms. For our cal-culations we assume a booster mass of 2,800 to 3,000 kilograms, which gives a fuelfraction of 0.84 to 0.85.

ACKNOWLEDGMENTSThe authors would like to thank Joe Bermudez, Bob Dietz, Mike Elleman, LisbethGronlund, George Lewis, Ted Postol, Maxim Tarasenko, and Steven Zaloga for com-ments and useful discussions and Lara Lumpe and Tim McCarthy for providing uswith news clippings on the North Korean program. David Wright was supported by agrant from the MacArthur Foundation through the Defense and Arms Control StudiesProgram at M.I. T. Timur Kadyshev was supported by the Ploughshares Fund and theUnion of Concerned Scientists.

NOTES AND REFERENCES1. The computer calculations are performed by integrating the equations of motion ofthe missile on a round, non-rotating earth in the presence of an atmosphere (for thelevel of accuracy considered here, the effects of the earth's rotation on missile rangecan be ignored). The drag coefficient of the missile as a function of speed during boostphase is approximated by using measured values for the V-2 rocket. For additionaldetails see appendix B of Lisbeth Gronlund and David Wright, "Depressed TrajectorySLBMs,. Science & Global Security 3 (1-2) 1992, p. 101.

2. John W. Lewis and Hua Di, "China's Ballistic Missile Programs: Technologies,Strategies, Goals,. International Security 17, fall 1992, p. 32; Joseph Bermudez, Jr.,"New Developments in the North Korean Missile Programme,. Jane's Soviet Intelli-gence Review, August 1990, p. 343; Joseph Bermudez, Jr., "Ballistic Ambitions Ascen-dant,. Jane's Defence Weekly, 10 April 1993, p. 20.

3. Several sources report that North Korea acquired the Scud-Bs from the SovietUnion. Steven Zaloga states they were acquired in the mid-1970s ("Ballistic Missiles inthe Third World,. International Defense Review, November 1988, p. 1423) (see also Y.A.Koptev, "How to Save the Russian Rocket Industry,. Rossiyskiye Vesti [Moscow], 17August 1993, p. 7 in FBIS-SOV-93-158, 18 August 1993, p. 9). Bermudez reports theywere acquired from Egypt in the early 1980s ("Ballistic Ambitions.).

4. Throughout this paper, we follow the naming convention for North Korean mis-siles from Bermudez, "Ballistic Ambitions,. with the replacement of "NKScud. for"Scud. to distinguish them from variants of the Soviet Scud missiles.

5. Bermudez, "Ballistic Ambitions..

6. In addition, Bermudez ("New Developments.) states that one of the design param-

--

An Analysis of the North Korean Nodong Missile-

eters for the DF-61 program in the mid-1970s was a heavy-gauge skin to protect themissile from damage during transport and handling, since the missile was to bemobile. This report suggests that the North Koreans would use steel for their othermobile missiles as well.

7. Bermudez, "New Developments.-

8. See for example, George P. Sutton, Rocket Propulsion Elements (fifth edition) (NewYork: John Wiley and Sons, 1986) p. 40.

9. Duncan Lennox, "Inside the R-17 'Scud B' Missile,- Jane's Intelligence Review(July 1991), p. 302; Warren Lenhart and Todd Masse, Persian Gulf War: Iraqi ScudBallistic Missile Systems, Congressional Research Service Report 91-173 F, 14 Febru-ary 1991, p. 6; "Race To Find Iraq's 'Scuds' -Jane's Defence Weekly, 26 January 1991, p.91; Bermudez, "Ballistic Ambitions-; Thomas Cochran, et al., Nuclear Weapons Data-book Volume N: Soviet Nuclear Weapons (New York: Ballinger/Harper and Row, 1989),p.221.10. W. Seth Carus and Joseph S. Bermudez, Jr., "Iraq's al-Husayn Missile Pro-gramme," Jane's Soviet Intelligence Review, May 1990, p. 204; Duncan Lennox, "Iraq's'Scud' Programme,- Jane's Defence Weekly, 2 March 1991, p, 301.

11. Sam Jameson, "North Korea Test Missile Able To Reach Japan," Washingtonnmes, 12 June 1993, p. A13; Bermudez, "Ballistic Ambitions,"

12. Reported range/payload combinations are 500 kilometers/700 kilograms (Bermu-dez, "Ballistic Ambitions-); 550 kilometers/600 kilograms (Milavnews, March 1991, p.23); 640 kilometers/680 kilograms (Steven Emerson, "The Postwar Scud Boom," WallStreet Journal, 10 July 1991).

13. Our estimate of the CEP for the separated warhead agrees well with estimatesgiven for the Iraqi aI-Hussein missile: the Congressional Research Service gives avalue of one to 1.5 miles (1,600 to 2,415 meters) (Lenhart and Masse, Persian Gulf War,p. 6) and Jane's Defence Weekly reports 1,600 to 3,200 meters ("Race To Find Iraq's'Scuds' "). This estimate also appears to be consistent with data presented in theunclassified pages of the U.S. Army report Patriot Performance Assessment in DesertStorm Roadmap, 15 July 1992, p. 10, which suggests that the predicted impact point ofan incoming aI-Hussein warhead varied by two to three kilometers during its reentry.

14. The durability of the engine may limit how much the amount of fuel may beincreased beyond that carried by the Mod-C. Increasing the fuel increases the length oftime the engine is burning and therefore subjects the engine to greater heating, whichthe cooling system on relatively simple engines like the Scud may not be able to with-stand without modifications. The configuration of the aI-Hussein may suggest that thebum time, and thus the fuel, cannot be increased by more than about 25 percent. Iraqalso reportedly built a missile called the aI-Abbas with 40 percent more fuel than theScud-B and a range of 900 kilometers with a 350 kilogram payload (Lennox, "Iraq's'Scud' Programme"). However, it was not used in the 1991 Gulf War and there are indi-cations that it may have had operational difficulties.15. According to figure 2, reducing the structural weight of the Mod-C by roughly 450kilograms, which would result from replacing the steel body with aluminum, wouldgive a range of less than 600 kilometers with a one tonne payload,16. Nan Yong-Chin, "DPRK's Advanced Weapons Analyzed," Nuclear Developments, 31May 1991, p. 7 (original: Hanguk Ilbo [Seoul], 8 February 1991, p. 8); Bermudez, "Bal-listic Ambitions."

56 Wright and Kadyshev

17. "DPRK Reportedly to Build New Scud Missile,. Nuclear Developments, 24 June1991, p. 7 (original: Yonhap [Seoul], 3 June 1991); Emerson, "The Post-war ScudBoom.; Gerald Seib, "Missile Race in Mideast, Along With Nuclear Issue, Raise Con-cern,. Wall Street Journal, 8 June 1992, p. A6. Libya will reportedly pay $7 millioneach for the missile (Bill Gertz, "Libya May Buy North Korean Missiles,. Washington7Imes, 4 June 1991, p. 4).

18. Reuters wire service, 18 April 1993.

19. Yonhap (Seoul), 14 July 1993 in FBIS-EAS-93-134, 15 July 1993, p. 18; Jon Wolfs-thaI, "U.S. Prods Israel to Halt Talks with North Korea on Missile Sales,. Arms Con-trol Today, September 1993, p. 24.

20. David Fulghum, "Mideast Nations Seek to Counter Air Power,. Aviation Week andSpace Technology, 7 June 1993, p. 77; David Sanger, "Missile is Tested by NorthKorea,. New York 7Imes, 13 June 1993, p. 7.

21. Jack Anderson and Michael Binstein, "North Korea: Loose Nuclear Cannon,.lAEA(International Atomic Energy Agency) Daily Press Review, 31 March 1992, p. 12.

22. Bill Gertz, "Iran-Bound Mystery Freighter Carried Parts for Missiles,. Washington7Imes, 16 July 1992, p. A3.

23. C.W. Lim, "North Korean Tells of Nuclear Accident,. Washington 7Imes, 25 August1993, p. AI. In 1990, construction of two bases for intermediate-range missiles wasreported near the Demilitarized Zone (Bill Gertz, "Libya May Buy North Korean Mis-siles,. Washington nIneS, 4 June 1991, p. 4). Guam is 4,000 kilometers from NorthKorea.

24. Bermudez ("Ballistic Ambitions.) notes reports of a test failure in 1991 and a suc-cessful test (although not to full range) somewhat later. Other reports of a successfultest prior to May 1993 include Gertz, "Libya May Buy North Korean Missiles"; 7bng-aIlbo (Seoul), 25 August 1991 in FBIS-EA, 26 August 1991; "Defense Ministry Says MayNodong-1 Test Successful" Yonhap (Seoul), 24 June 1993 in FBIS-EAS-93-120, 24 June1993, p. 19; "North Korea Seen Successful in Test of Nodong-1 Missile,. AerospaceDaily, 29 June 1993, p. 538. For reports that no previous tests had occurred see JoannaPitman, "Worried Thkyo May Buy Patriot Defense," 7Imes (London), 12 June 1993;Sanger, "Missile Is Tested.. Reports that a Nodong exploded on the launch pad in May1990 (Bill Gertz, "General Spotlights North Korean Threat," Washington 7Imes, 15September 1993, p. A6) apparently refer to U.S. satellite evidence (Bill Gertz, "NorthKorean Missile Apparently Blows Up,. Washington 7Imes, 5 July 1990). These reportsare believed to refer to a new missile that may have been a Nodong prototype.

25. Reuters wire service, 11 June 1993; Andrew Katell, "North Korea Suspends Moveto Quit Nuclear Arms Pact," Boston Globe, 12 June 1993, p. 4; Jameson, "North KoreaTest Missile"; Seoul KBS-I Radio, 1 June 1993 in JPRS-TND-93-018, 14 June 1993, p.3; Yonhap (Seoul), 15 June 1993 in FBIS-EAS-93-113, 15 June 1993, p. 28; "NorthKorea Seen Successful in Test of Nodong-l Missile," Aerospace Daily, 29 June 1993, p.538; Aviation Week and Space Technology, 5 July 1993, p. 17; Yonhap (Seoul), 14 July1993 in FBIS-EAS-93-134, 15 July 1993, p. 18; Gertz, "General Spotlights Threat."

26. "Japan Says North Korea to Finish Missile Soon,. Reuters wire service, 11 Novem-ber 1993.

27. "Defense Ministry Says May Nodong-l Test Successful. Yonhap (Seoul), 24 June1993 in FBIS-EAS-93-120, 24 June 1993, p. 19; Sanger, "Missile Is Tested."

An AnalysIs of the North Korean Nodong Missile~-_.~

28. "Official Confirms North's Mobile Test Launch," Choson Ilbo (Seoul) 3 December1993, p. 1 in FBIS-EAS-93-231.

29. Gertz, "General Spotlights Threat."

30. Kyodo (Tokyo), 12 June 1993 in FBIS-EAS-93-112, 14 June 1993, p. 5; UPI wireservice, 14 June 1993; Yonhap (Seoul), 15 June 1993 in FBIS-EAS-93-113, 15 June1993, p. 28.

31. Range estimates appear to be based on the size and fuel capacity as derived fromimages of the missile. References include: A. Platkovskiy, "North Korea Nuclear Pro-gram Viewed," Nuclear Developments, 12 September 1991, p. 24 (original: Komsomol-skaya Pravda [Moscow], 24 August 1991); Kyodo (Tokyo), 20 September 1991 inProliferation Issues, 29 October 1991, p. 7; Paul Lewis and David Silverberg, "WestWorries China Will Sell Missiles," Defense News, 16 March 1992, p. 1; Gerald Seib,"Missile Race in Middle East Countries Despite U.S. Efforts to Stall Buildup," WallStreet Journal, 8 June 1992, p. A6; Katell, "Nor'- Korea Suspends Move"; AviationWeek and Space Thchnology, 5 July 1993, p. 17; Martin Sieff, "Israel to Woo NorthKorea to Keep Missiles From Iran," Washington nmes, 15 June 1993, p. A8.

32. "DPRK Reportedly Boosts Range of Nodong, Sells to Iran," Yonhap (Seoul), 8 June1993, in JPRS-TND-93-018, 14 June 1993, p. 27; Yi Chang-sun, "Japan's Sense ofTer-ror for Nodong-l," Sinmun (Seoul), 13 June 1993, p. 7 in FBIS-EAS-93-112, 14 June1993, p. 31; "North Korea Seen Successful," Aerospace Daily.

33. Since the combustion chamber and nozzle of the Scud-B engine are roughly 0.4meter in diameter there is room to cluster four of them in the 1.2 to 1.3 meter bodydiameter of the Nodong (estimates of engine size were derived from photographs pro-vided by Israeli journalist Reuven Pedatzur of an Israeli Defense Force display of al-Hussein wreckage in Tel Aviv).

34. The fuel and oxidizer in these missiles were each stored in a single large tank; theSoviet design used a single large turbo pump that fed fuel to all four engines, but theChinese version used a separate small pump for each engine (Hua Di, private commu-nication).35. Hua Di, private communication.

36. Nan Yang-Chin, "DPRK's Advanced Weapons Analyzed" gives values of 15.1meters in length and 1.3 meters in diameter. The article states a range of 600 kilome-ters, which does not appear to be consistent with the dimensions. The author may haveassumed the missile was a Mod-C rather than a Nodong, the author may haveintended a range of 600 miles rather than 600 kilometers. Yonhap (Seoul), 14 July1993, in FBIS-EAS-93-134, 15 July 1993, p. 18 gives values of 15.8 meters in lengthand 1.2 meters in diameter.

37. We assume here that the same fuel and oxidizer are used as in the Mod-B andMod-C and are used in the same proportions.

38. Ed Blanche, Associated Press wire service, 22 October 1993; Reuters wire service,28 October 1993.

39. It is possible North Korea might test the missile with a very small payload to sug-gest a greater range capability than it actually had.

40. Lewis and Di, "China's Ballistic Missile Program."

41. Lewis and Di, "China's Ballistic Missile Program." China switched to stabilized

158

Wright and Kadyshev

platform guidance in the late 1970s with the development of the DF -5/CSS4.

42. Yonhap (Seoul), 14 July 1993, in FBIS-EAS-93-134, 15 July 1993, p. 18.

43. Chicago Sun nInes (20 April 1993, p. 46) reported that Iran stopped shipping oil toNorth Korea in 1992 after the Scuds it had received (presumably Mod-Bs or Cs) werefound to be defective.

44. Bermudez, "Ballistic Ambitions.; Tony Emerson, "Nuclear Nightmare," Newsweek,29 November 1993, p. 20.

45. Lewis and Di, "China's Ballistic Missile Programs."46. This calculation assumes the first stage is made of steel to provide the structuralstrength to support the second stage, and has a fuel fraction of 0.8. The second stagefuel fraction is 0.77. If the fuel fraction of the second stage could be increased to 0.80 to0.81 (for example, by using aluminum alloy for the body) the range would increase to1,950 to 2,050 kilometers.

47. Some Japanese military officers are reportedly skeptical that North Korea's cur-rent technical and industrial capabilities are sufficient to produce such a missile("North Korean Missile Eyed With Skepticism," Aviation Week and Space Technology,18 October 1993, p. 101).48. Following the reported test of the Nodong missile in May of this year, Israel heldthree meetings with North Korea to discuss economic aid in return for commitmentsnot to sell the missile to countries in the Middle East. Israel suspended the meetingsunder pressure from the U.S. (David Hoffman, "Israel Agrees to Suspend Contactswith North Korea," Washington Post, 17 August 1993; Jon Wolfsthal, "U .S. Prods Israelto Halt Talks with North Korea on Missile Sales,. Arms Control Thday, September1993, p. 24). In addition, Japan has begun to consider developing or buying tacticalmissile defenses (David Sanger, "New Missile Defense In Japan Under Discussion withU.S.," New York llmes, 18 September 1993, p. 1; Barbara Opall and Naoaki Usui,"Japan, U.S. Pursue Ballistic Missile Defense," Defense News, 4 October 1993, p. 3).

49. Philip Finnegan, "Gulf War Exposes Effectiveness of Countermeasures," DefenseNews 12 August 1991, p. 1; Theodore Postol, "Lessons of the Gulf War Experience withPatriot,. International Security 16 (3), winter 1991/92, p. 119; George Lewis and The-odore Postol, "Video Evidence on the Effectiveness of Patriot During the 1991 GulfWar," Science & Global Security 4 (1), 1993, p. 1.

50. A bent nosetip on the MaRV produced the same type of lateral accelerations thatthe Nodong warhead would experience (see for example Matthew Bunn, "The NextNuclear Offensive," Technology Review, January 1988, p. 28.

51. A Nodong warhead with a ballistic coefficient of 72,000 N m-2 (1,500 lb ft.-2) wouldhave a speed 30 to 60 percent faster than the al-Hussein missile at altitudes of 10 to 30kilometers, resulting in lateral forces 70 to 250 percent greater.

52. Steve Fetter, "Ballistic Missiles and Weapons of Mass Destruction," InternationalSecurity 16, summer 1991, p.5.

53. Steve Fetter, George Lewis, and Lisbeth Gronlund, "Why Were Scud Casualties SoLow?" Nature 361, 28 January 1993, p. 293.

M. Duncan Lennox, "Inside the R-17 'Scud B' Missile,. Jane's Intelligence Review, July1991, p. 302. This article contains a number of detailed photographs of the missile,which were used to derive some of the dimensions of the missile used below.

An Analysis of the North Korean Nodong Missile~ --Q 55. The export version of the Scud-B, the R-17E, may use UDMH fuel with N2O4

rather than IRFNA as the oxidizer. The density of N2O4 is 1.44 g cm-3 and the opti-mum mass ratio (N204/UDMH) is about 2.6. Four tonnes of propellant would thereforehave a volume of 3.4 m3. Using N204 could increase the specific impulse by several percent compared to using IRFNA. Assuming N 20 4 is used instead of IRFNA would makeonly small changes in our model and would not affect our conclusions. In particular,the estimated length of the Nodong would increase by 0.5 meters, and the booster masswould increase by about GO kilograms.

56. IRFNA III-A is 83.4 percent HN03 and 14 percent N02, and IRFNA IV HDA is54.3 percent HN03 and 44 percent N02. The optimal mass ratios (IRFNA/UDMH) forthese two oxidizers are 3.13 and 2.85, respectively. The density of UDMH is 0.789 gcm-3 and for IRFNA (Ill-A) is 1.57 g cm-3 (Dieter K. Huzel and David H. Huang, Mod-em Engineering for Design of Liquid-Propellant Rocket Engines [Washington DC:AIM, 1992) p. 20).57. This ratio agrees with Caros and Bermudez's claim that the aI-Hussein tankswere increased by 0.45 and 0.85 meters over the Scud-B (Carus and Bermudez, "Iraq'sal-Husayn Missile Programme-). The ratio may be increased above optimum to keepthe combustion chamber from overheating.

58. These quantities are related by the expression T = golspJ.!/tb' where T is thrust innewtons, go is 9.8 m sec-2, IBP is specific impulse in seconds, Mp is the propellant massin kilograms, and tb is the bOoster bum time in seconds.

59. Carus and Bermudez, "Iraq's al-Husayn Missile Programme.-

GO. See equations (D-3) and (D-4) in Gronlund and Wright, "Depressed TrajectorySLBMs- Science & Global Security 3 (1-2). It is equivalent, but simpler, to do this cal-culation using the two components of burnout velocity as we do here rather than usingthe magnitude and direction of the burnout velocity, which is more standard.

61. We ignore the other contributions to guidance and control errors, such as errors inthe burnout height, which will give small contributions to the total.

62. Net lift forces can occur even if the reentry vehicle is spinning (see, for example,D.H. Flatus, "Dispersion of Spinning Missile due to Lift Non-Averaging,- AlAA Journal15, July 1977, p. 909).

63. Using a constant perturbation whose magnitude is chosen to give the observed dis-persion is a standard technique used to estimate the effects of other atmosphericeffects on reentry dispersions (see, for example, Doreen H. Daniels, Ballistic Correla-tion Altitudes for Reentry Winds and Nonstandard Air Densities (Dahlgren, Virginia:U.S. Naval Weapons Laboratory, April 1965) NTIS,AD-614-710, p. 9).

64. This is illustrated by the difference in CEPs of the two cases calculated for theMod-C missile. While increasing the reentry speed increases the lift forces acting onthe warhead, which are proportional to y2, it also reduces the time these forces act onthe warhead, which leads to smaller dispersions. The calculations for the two Mod-Ccases assume that the magnitude of the lift coefficient is the same in both cases; as dis-cussed below, however, increasing the ballistic coefficient may give rise to additionallift forces.

65. For a discussion of some of these issues, see D.W. Hall and D.T. Nowlan, "Aerody-

namics of Ballistic Reentry Vehicles with Asymmetric Nosetips,- Journal of Spacecraftand Rockets 15, January/February 1978, p. 55; W.S. Kobayashi and J.L. Saperstein,"Low-Temperature Ablator Tests for Shape-Stable Nosetip Application on Maneuvering

160 Wright and Kadyshev

Reentry Vehicles,. in Thermophysics of Atmospheric Entry, editor T.E. Horton (NewYork: American Institute of Aeronautics and Astronautics, 1982) p. 148.

66. J.R. Sellars, "Reentry and Recovery,. in H.S. Seifert and K. Brown, Ballistic Mis-sile and Space Systems (New York: John Wiley & Sons, 1961) p. 452.

67. This value of~ is large because the weight of the reentering warhead and attachedmissile body is large. It assumes a value of the drag coefficient of 0.2

68. The drag and ballistic coefficients are related by ~ = goml(C~), where Cd is thedrag coefficient, go is the acceleration of gravity, m is the mass of the warhead, and A isthe cross-sectional area of the warhead perpendicular to the direction of motion.

69. Frank J. Regan, Re-entry Vehicle Dynamics (New York: American Institute of Aero-nautics and Astronautics, 1984) p. 230.

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